[RRFC] Support a src package companion to main one

Motivation ("The Why")

There is a significant trend in thinning the size of npm packages by removing:

1. source files (in case it's transpiled)
2. test files
3. build files
4. docs (including GOVERNANCE and CODE_OF_CONDUCT files)

However it's important that those files are available along side the release in an immutable fashion (in case the main repo is unavailable for example) as they will be needed to recreate the project.

This is a common concept for Linux distributions as they ship both source and binary package as part of the same release.

Example

I propose we add another field in package.json:

{
  ...
  "source" : [
    "src",
    "build",
    ...
   ]
}

And then a command npm source <pkg> that downloads the source tarball.
A config lag that always installs the full tarball inside node_modules would be nice.

Start by reviewing the proposed package.json source field and the npm source command described in the issue. Define the package contents, source tarball behavior, and configuration for installing the full tarball; done would require an agreed design before implementation can begin.